This project is to reveal the major molecular bases for the function of KCNQ K+ channels (KCNQ1-5) that are important in the heart, brain, inner ear and epithelia. The aberrant functions of these channels are associated with cardiac arrhythmia, epilepsy, deafness, and gastric cancer. The importance of these channels is based on two prominent properties. First, all these channels, when expressed without auxiliary subunits, are activated by voltages at negative ranges (start activating around -60 mV). Being activated just above the resting membrane potential, the M-current through KCNQ2 and 3 in neurons reduces membrane excitability and acts as a brake to membrane discharge. Second, the association of the KCNE family K+ channel subunits, which radically alter gating, permeation and pharmacological properties of KCNQ1, determines the physiological role of KCNQ1. KCNQ1 associates with KCNE1 to form the IKs channel in the heart that regulates action potential duration, and with KCNE2 or KCNE3 to form the constitutively open background K+ channels in epithelia important for ion transport. What are the mechanisms underlying these properties? This is a long-standing question whose answer will provide the basis for the understanding and treatment of KCNQ associated diseases. We propose that the answer to this question lies in a novel mechanism for KCNQ1 activation. During voltage dependent activation in KCNQ1, the voltage sensor domain (VSD) moves in two steps, from the resting to intermediate and then to activated state; the channel pore opens at both intermediate and activated states of VSD, but the VSD-pore interaction differs at different states of VSD to alter channel gating, ion permeation and pharmacology. KCNE1 modulates channel function by suppressing the intermediate openings of the channel. In this project we wish to test various aspects of this hypothesis in three specific aims. 1) We wish to identify the structural motifs that are important for the VSD-pore interaction at intermediate and activated states, respectively. Mutations in KCNQ1 and KCNE1 are associated with long QT syndrome (LQT) that predispose patients to fatal cardiac arrhythmia, this study will reveal if some of the LQT mutations alter VSD-pore interaction. 2) We will examine if suppression of the intermediate and potentiation of the activated openings is the main mechanism for major functional changes upon KCNE1 association including the response to - adrenergic stimulation and inactivation gating. LQT patients with KCNQ1 mutations often experience symptoms of cardiac arrhythmia such as syncope and sudden death during exercise when -adrenergic pathway is stimulated. This study is therefore particularly significant for the understanding of molecular bases of LQT. 3) We wish to reveal if KCNE2 and 3 make the channel constitutively open by altering VSD activation, pore opening or VSD-pore interaction. We will identify if these channels are at intermediate or activated open states and their responses to drugs and cellular signaling molecules.